"He's taken The Guardian on a great digital journey and charted a course that's made it worldwide force."

Rusbridger said: “In global journalism, there are a handful of roles that have the capability to redefine our industry. I am privileged to have held one of those roles for 20 years, a period in which successful newspapers have become global content providers, reaching audiences in dramatically new and valuable ways.

“I am honoured to succeed the quite brilliant Liz Forgan as chair of The Scott Trust, preserving the independent editorial values and the long-term financial stability upon which our future depends. We have strong future leaders in place with unparalleled news and digital experience, and I know that our journalism will be in the best possible hands.”

He will replace Dame Liz Forgan as chair of the Scott Trust.

She said: “Alan has been the outstanding editor of his generation. Fully embracing the opportunities of the digital age, he has built on the best traditions of his distinguished predecessors, transforming the Guardian from a print-only national newspaper into the world's leading quality newspaper website.

“We are delighted that The Scott Trust and the wider Group will continue to benefit from his experience, overseeing the independent body that guarantees the editorial integrity and commercial future of the Guardian.”

Neil Berkett, chairman of Guardian Media Group, said: “Alan has set the standard for journalistic leadership in the digital age. His appointment to lead The Scott Trust coincides with rapidly rising readership, continued innovation and secure finances at the Guardian. His successor will inherit a global media organisation in very strong health and with clear prospects for further growth.”

Rusbridger told staff in an email today:

This is to let you know that next summer I will be stepping down as editor-in-chief of the Guardian before succeeding Liz Forgan as Chair of The Scott Trust when she reaches the end of her term in 2016.

In February I’ll have been editor for 20 years. It’s been quite an extraordinary period in the life of the Guardian. In February 1995 newspaper websites were, if they existed at all, exotic things: we were still four years off launching Guardian Unlimited. Since 1999 we’ve grown to overtake all others to become the most-read serious English language digital newspaper in the world.

When I assumed the editorship in 1995, the senior team at the Guardian was debating whether we should switch to using colour photography in the paper. (There were quite a few distinguished voices believing black and white was the proper métier for news.) Today we are doing our journalism in words, (colour!) pictures, video, data, animation, audio; on mobile and other platforms and in social … and every possible combination of the above.

The past two decades have been marked out by wonderful Guardian writing, photography, innovation and editing. There have been gruelling court battles, dogged campaigns and tough investigations. The Guardian – always the outsider – has won a global reputation for its willingness to fight for the right causes. We have strong future leaders in place with unparalleled news and digital experience. We have built up – and banked – a considerable financial endowment to secure future innovation and build on our quality journalism. The GMG Board is prepared to invest significantly in what we do because of the extraordinarily strong global position for which we (editorial, commercial and digital together) have fought and won.

Each editor is told – this is literally the only instruction – to carry the Guardian on “as heretofore”. That means understanding the spirit, culture and purpose of the paper and interpreting it for the present. All that is only possible because of the unique Scott Trust, set up in 1936 to ensure the Guardian survives in perpetuity.

Since 1936 the Trust has always appointed a chair from within – in every case a member of the Scott family or a former Guardian journalist or editor. I’ve felt very lucky to have Hugo Young and Liz Forgan beside me and/or guarding my back. The Trust is one of the most important liberal institutions in the world and I was very honoured to be asked to succeed Liz as Chair when she steps down in 2016.

But the best thing about working here – the thing I’ll miss most – are my colleagues. We are a team and the strongest of communities – one which includes our readers. The community includes people from all areas, in and outside editorial. The Guardian and The Observer are bursting with extraordinarily bright, talented, brave, kind, knowledgeable, resourceful, imaginative, thoughtful and delightful people. I know our journalism – and our “perpetuity” – will be in the best possible hands.

I am currently visiting the Guardian Australia team in Sydney, Melbourne and Canberra – another amazing Guardian success story – but I will be back in Kings Place on Monday and will talk to you then.

Rusbridger announced his departure shortly after 4pm on Twitter. Here's some Twitter reaction:

The Guardian holds open hustings to elect an editor. Staff vote for their fave. The Scott Trust (new chair: Rusbridger) ratifies decision.